A brake system of this type is disclosed in WO 91/18776. The prior art brake system is appropriate for anti-lock control and traction slip control. Traction slip control operations, as a type of active braking, do not require the application of the brake pedal. Therefore, the wheel brakes of the driven wheels must be filled by a pump. The prior art system uses the return pump of the anti-lock control device for this purpose. In order that a return pump of the non self-priming type can also be used to this end, a pressure accumulator is provided in a suction line from the suction side of the pump to the master brake cylinder. The objective of the pressure accumulator is to furnish a pilot pressure to the suction side of the return pump during traction slip control operations. However, this arrangement is also appropriate to precharge a self-priming return pump. A switch valve is interposed between the pressure accumulator and the suction side of the return pump which will connect the pressure accumulator to the suction side of the return pump only when traction slip control shall be performed.
It is inconsequential to the present invention whether the return pump is of the self-priming type or requires precharging. The basic problem of using high-pressure pumps for brake systems is as follows. Before it is possible to achieve a braking effect on the wheel brakes, it is necessary to supply a defined volume into the brake line in order to overcome the clearance in the wheel brakes. Although return pumps are capable of developing high pressure, the volume of fluid delivered is so small that it takes a long time (at initial low pressures) to effect braking. To provide the fluid volumes necessary to overcome the clearance of wheel brake cylinders, a return pump (be it of the self-priming or the non self-priming type) is therefore only marginally adequate. This is because a large volume of fluid rather than high pressure is necessary to this end.
An object of the present invention is to equip a brake system of the above-mentioned type by simple means so that braking effects can quickly be achieved in active braking operations by use of the return pump.
The principle of the present invention is that the return pump acts upon a small effective surface of a stepped piston and, due to its high supply pressure, displaces the stepped piston, with the result that the large effective surface of the stepped piston displaces a pressure fluid volume greater than the one delivered by the return pump. Because overcoming the clearance of the wheel brakes does not necessitate high pressure, the large surface of the stepped piston may be rated considerably greater than the small surface. When a high pressure is required in the wheel brakes after this precharging action, the return pump can deliver fluid through the pressure line directly into the wheel brakes and generate a high pressure also with a low volume flow. However, in order that initially the stepped piston is displaced, a means is provided in the pressure line in parallel to this pressure-volume converter. This means initially accumulates the pump pressure.
A pilot pressure valve is suitable in this arrangement which has the advantage that it opens permanently at a constant pressure difference. Its opening pressure is rated so that the back pressure which develops is just sufficient to fill the wheel brake cylinders of the wheels subjected to active braking.
Spring bias of the stepped piston towards the pressure side of the pump causes an automatic charging of the volume accumulator when the return pump is deactivated.
In order that the fluid volume disposed between the pressure side of the pump and the stepped piston can be displaced when the volume accumulator is charged, it is advisable to provide a throttle device in parallel to the stepped piston. The enclosed fluid volume can be discharged through the throttle device towards the brake line.
A seat valve which closes the connection between the brake line and the stepped piston as soon as the stepped piston has overcome its stroke for filling the wheel brakes prevents a return movement of the stepped piston during active braking operations. Because the valve is so rated that it has an effective surface in the opening direction which is considerably smaller than the small stepped piston surface, the braking pressure which developed in the brake line due to the return pump is prevented from returning the stepped piston towards the return pump. Further, it is also prevented that the entire delivery volume of the return pump is discharged into the volume accumulator.
A particularly space-saving solution is achieved when the throttle (that is in parallel to the stepped piston) is positioned inside the stepped piston for relief of the pressure line between the return pump and the stepped piston. Besides, it is easier for a bore of this type to extend through a piston than through a valve block.